Electrolytic tin plate production



United States Patent Otlice 3,052,726 Patented Nov. 6, 1962 3,062,726 ELECTROLYTIC TEN PLATE PRODUCTEQN Uno T. Hill, Gary, Ind, assignor to Inland Steel Company, Chicago, 115., a corporation of Deiaware No Drawing. Filed Aug. 21, 1953, Ser. No. 756,280 9 Claims. (Cl. 204-40) This invention relates to the production of tin plate. It is more specifically concerned with the preparation of non-porous tin plate by processing low carbon steel sheet in an electrotinning process.

Because tin plate has unique properties resulting from its combining the strength of steel with the corrosion resistance of tin, tin plate has been found to be an excellent material of construction in the fabrication of numerous items, especially containers for foods and other products. In preparing tin plate for application in the packaging of corrosive materials, it is essential that surface imperfections be minimized and that a substantially non-porous surface be produced. Because tin coatings do not provide a sacrificial action by yielding to corrosion more than steel, if tin-plated steel has present pin holes or porosity in the coating the base metal will corrode while leaving the tin coating intact. It is therefore essential that nonporous tin coatings be provided in order that corrosion of the base metal plate will be minimized.

In tin plating steel sheet several techniques are available. Although tin coating of steel sheet by hot dipping techniques has long been used in the steel making industry, the production of steel sheet by continuous techniques has resulted in the wide-spread use of continuous processes wherein tin is electrolytically deposited on sheet steel to produce tin plate having a superior uniformity in tin coating together with close control of the tin coating weight. Because of the advantages of electrotinning, a predominant portion of tin plate is produced by means of this process.

According to this invention the corrosion resistance of electrolytically deposited tin coating is substantially improved for a given coating weight by applying the tin coating in two operations. The primary coat is first deposited upon the base metal to provide a coating weight, per side, of not more than about .08 and preferably .04 to .06 pound of tin per base box. The tin plate with the primary coating is then fused by melting and quenching in accordance with conventional tin fusion techniques. The tin plate is then chemically treated with an etchant solution. After the treated tin plate has been rinsed to remove the etchant solution, it is returned to the electrotinning process for application of a second coating of tin. Thereafter the double coated tin plate can be finished conventionally.

In accordance with this invention it has been found that double coated tin plate has superior corrosion resistance, for a given finished coating weight, than tin plate which is singly coated by a conventional electrotinning process. In addition, desired equivalent corrosion resistance can be obtained by this invention by employing tin coatings having a smaller weight per unit area.

To specifically illustrate the instant invention, an 85# (.0091" thick) black plate base metal was used. This base metal had substantially the following composition.

Carbon .07% max. Manganese .32.40%. Phosphorus .015%. Sulphur 032%. Silicon Residual. Copper .19%.

The black plate base metal was prepared employing conventional hot rolling, cold reduction, annealing, and temper rolling basic processing. Sufficient coupons 5" x 8" were prepared in order that coating weight, porosity, rust resistance, lacquer adherence, and solderability could be obtained. For evaluation purposes the coupons were processed at the same time according to an established sequence of operations which included the following steps.

(1) Pickling for 5 seconds in 3% vol. aqueous solution of H at 350 F.

(2) Rinsing with water.

(3) Dipping in .2 vol. percent aqueous solution of H 80 to remove any oxide coating.

(4) Dipping in an aqueous solution of sodium hydroxide having a concentration of 0.1 gm./liter.

(5) Plating a 5 x 6" section of the coupon for 12 seconds employing a current density of 20 amperes per square foot and an alkaline electrolyte containing 1.5 oz./ gallon of sodium hydroxide, 14 oz./ gallon of sodium stannate, and a metallic tin concentration of 5.6 oz./ gallon to provide an initial coating weight of 0.07 pound per base box.

(6) Rinsing with Water.

(7) Fusing by induction at a temperature of about 450 F. and water quenching.

(8) Dipping for 5 seconds in a 1% solution of hydrofluoric acid at room temperature.

(9) Rinsing in water.

(10) Plating for 23 seconds in the alkaline electrolyte described above employing a current density of 20 am peres per square foot to provide a final coating weight of 0.16 pound per base box.

(11) Rinsing in water.

(12) Fusing by induction at a temperature of about 450 F.

(13) Quenching in water.

The normal production tin plate was plated for 35 seconds at 20 amperes per square foot employing the procedure outlined in steps 1-5 above followed by water quenching. The results of this investigation are tabularly summarized in Table I.

The solderability test is carried out by folding lengthwise a 1 x 3" test specimen of tin plate so that the' The bottom of the strip is thenedges overlap dipped in palm oil and the bottom immersed in a lead bath to a depth of 1" for one minute, with the tempera ture of the bath held at a constant temperature of 700 F. The lead will flow upwardly along the capillary formed by the overlapping edges and the height of the capillary rise of the bath is reported in M increments.

Thus a rise of 2 or a solderability of 32 is Experience has proven that a,"

maximum for the test. solderability of 9 or less is undesirable while other factors such as poor adhesion of the solder bond may be objectionable.

The porosity test is a ferricyanide print of the tin plate surface employing a gelatinized printing paper dipped in the ferricyanide solution employing sodium chloride I which is held to the surface of the plate for a specified time. Iron particles, iron oxide, or a pin hole in the plate will show up as a blue colored area or spot.

The corrosion test is carried out in an ASTM corrosion cabinet employing in this instance Lake Michigan water rather than salt water for the corroding media. In this test a tin plate specimen is retained within the cabinet for 22 hours under conditions of 100% relative humidity at a temperature of 95 F. The specimen, after the required residence time in the cabinet has been completed, is removed and the specimen visually evaluated within a two hour period after its removal.

The lacquer adherence test is carried out by lacquering the tinned specimen With a phenolic type lacquer, and after baking the adhesion is tested by observing whether the lacquer adheres to the plate when a container end is punched from the specimen.

From the data in Table I it will be seen that the characteristics of the double coated tin plate are superior to those provided by tin plate employing conventional production techniques utilizing a single electro-tinning step. It will be seen from these data that the solderability values are as high as possible and the lacquer adherence was rated as acceptable. The advantages of the instant invention are further emphasized by the superior corrosion resistance obtained for the tin plate produced in accordance with this invention as well as the substantial reduction in porosity which obtained.

Although in the foregoing test the corrosion resistance of the tin plate produced was far superior to a conventionally produced tin plate having the same coating weight, tin plate can also be produced in accordance with this invention having satisfactory corrosion resistance with lesser coating weight than would be necessary in providing tin plate by conventional production techniques.

In carrying out the instant invention, the various unit steps employed in conventional electrolytic tin plating methods can be employed utilizing acid or alkaline electrolytes containing a variety of tin salts. The steps normally employed include cleaning the sheet metal to be processed with an alkaline detergent cleaning solution, pickling the clean strip with a suitable pickling agent such as hot sulphuric acid, water rinsing the strip, and thereafter passing the pickled sheet steel through the electrolytic tinning unit. In these units current densities of between to 30 amperes per square foot are employed although current densities outside this range can be employed if desired. The plated strip is rinsed with water to remove the residual electrolyte. Thereafter the electrolytic tin plate is passed through a fusion unit wherein the electrodeposited tin is melted by means of an electrical resistance unit, induction heating, gas heated melting units or the like; and subsequently quenched in order to give the electrolytic tin plate a brilliant luster. In the finishing of the tin plate a controlled oil film is deposited on the surface of the electrolytic tin plate in order to improve its handling properties in the succeeding inspection and classification operations. In these various steps conventional equipment is utilized for carrying out the function of each operation.

In order to carry out the two step plating technique of this invention, the tin strip, after having the initial coating of tin electrodeposited, is processed after fusion and quenching in an intermediate chemical treating operation wherein the once coated tin strip is passed through a chemical etching solution which is characterized by its ability to rapidly react with iron and iron compound particles to effect their conversion into reaction Products which can be readily rinsed from the surfaces of the tin plate. The tin coating, of course, must be substantially unaflected by the intermediate etching step. The preferred etchants such as strong inorganic acids will have no deleterious effect on the tin coating because of the minimal time of immersion in the etchant solution and concentration of the solution. Immersion times of 2-6 seconds generally are used; however, longer or shorter periods of immersion can be used depending upon the process conditions.

It is postulated that in order to produce the corrosion resistant electrolytic tin plate having reduced porosity in accordance with this invention, it is necessary to remove all foreign particles such as metallic iron and compounds of iron from the surfaces of the base metal sheet. These particles are so tightly held to the metal surfaces by valence forces residence in the base metal that ordinary cleaning methods employed in preparing a steel base metal sheet for subsequently electrotinning fail to dislodge them. When the base metal is tinned in the electrolytic tinning process the particles are covered over by the electrolytically deposited tin. These particles however diffuse and tend to migrate to the surface of the tin coating when the layer of tin is fused to produce a mirror-like finish. The extent of this fusion is governed by the viscosity of the fused tin coating, its thickness, and the duration of the fusion period. Thinner coatings of tin will have a greater concentration of these particles than thicker coatings because the distance of diffusion is less than in the thicker coatings in order to reach the outer surface of the tin coating; and the tin itself is less viscous than in a thicker coating because the heat necessary for fusion had a shorter distance of plate to penetrate. Since porosity tests as commonly conducted simply determine the amount of iron on the surface of the plate, the thinner coating is classified as being more porous than the thicker coating. The foreign particles which produce this deleterious effect result from the various processing steps involved in the production of the sheet steel such as pickling, rolling, and others and consist primarily of finely divided metallic iron, iron oxide, iron sulfate, iron hydroxide, and other like particles which remain on the surfaces of the base metal after conventional cleaning techniques have been used. It is to be understood, however, that this postulation is non-limiting and is suggested as a possible mechanism to explain the process of this invention.

Although an aqueous solution of hydrofluoric acid was employed in the illustrative embodiments, it is obvious that other equivalent etching expedients can be employed. These include not only aqueous solutions of inorganic acids such as hydrofluoric, fluophosphoric, sulfuric, hydrochloric, and the like but also other organic or inorganic compounds which will rapidly react with iron compounds to effect their conversion into water-soluble or water-insoluble compositions which can be readily rinsed from the surface of the initially plated tin plate. The concentrations of the etchant solutions will depend upon the strength of the reagent. Generally if inorganic acids are used, 0.1 to 10.0 vol. per cent solutions will be satisfactory.

In order to provide a continuous operation, it is preferable to employ a pair of electrotinning units in tandem with the necessary auxiliary pickling and rinsing equipment suitably positioned in relation therewith in order to permit a continuous operation. If a continuous operation is not necessary a single tinning unit can be employed in which the initially plated strip is recoiled subsequent to the etching treatment of the tin plate and thereafter passed through the electrotinning operation to electrolytically deposit the second layer of tin thereon in order to effectuate the objectives of the instant invention. After a second electrotinning step the tin plate is then processed in accordance with conventional finishing operations eliminating the intermediate treating step required to effect the removal of the undesirable iron or iron compound particles. In this finishing operation the double coated tin plate is generally fused and quenched to provide a bright luster. These steps however can be eliminated if a matte finish is desired.

The steels preferably utilized for tin plate production generally are made by the open hearth process and are classified as soft carbon steels having a maximum carbon content of about .O7%. Other steels however can be employed if desired. The steels used are processed into strips of relative thin gages employing the conventional hot rolling, cold reducing, annealing, and temper rolling procedures.

The electrolytic tin plate produced in accordance with this invention can be prepared in any coating weight. In general however, it is preferred that finished total coating weights for both sides of 0.25-1 pound of tin per base box be prepared in accordance with this invention. If desired, however, heavier coating weights can be produced. In order to provide the advantages of this invention a coating weight, per side, of less than .08 and preferably .04-.06 pound of tin per base box is initially electrolytically deposited on the base metal in order to provide for the beneficial effects of this invention. In the second electrotinning step, sufficient amounts of tin are electrolytically deposited in order to provide a finished tin plate having the desired coating weight.

It will be apparent from the foregoing discussion that variations in the various operations employed in the sub ject electrotinning process can be employed by those skilled in this art without departing from the scope of this invention. Various pickling techniques, cleaning and rinsing procedures, electrotinning operations, and fusion and quenching processes can be employed. In addition alternative chemical etching techniques will be apparent which will be within the scope of this invention. Accordingly, it is to be understood that the two step, electrolytic tinning process of this invention is to be limited only as defined by the appended claims.

I claim as my invention:

1. A method for reducing the porosity of tin coating and increasing the corrosion resistance of a tin-coated steel base sheet which consists essentially of coating a steel base sheet with a thin, first layer of tin to produce a tin-coated, steel base sheet, fusing said first layer of tin, quenching the tin-coated, steel base sheet, applying to the tin-coated surfaces of the base sheet a chemical etchant characterized by its ability to rapidly react with iron and iron compounds and without deleteriously affecting said tin-coated surfaces and selected from the group of acids consisting of hydrofluoric, fluophosphoric. sulfuric and hydrochloric, and coating the tin-coated steel base sheet with a second layer of tin.

2. A method for reducing the porosity of tin coating and increasing the corrosion resistance of a tin-coated steel base sheet which consists essentially of electrolytically coating a steel base sheet with a thin, first layer of tin to produce a tin-coated, steel base sheet, fusing said first layer of tin, quenching the tin coated, steel base sheet, applying to the tin-coated surfaces of the base sheet a chemical etchant characterized by its ability to rapidly react with iron and iron compounds and without deleteriously affecting said tin-coated surfaces, and selected from the group of acids consisting of hydrofluoric, fluophosphoric, sulfuric and hydrochloric, and coating the tincoated steel base sheet with a second layer of tin.

3. A method for reducing the porosity of tin coating and increasing the corrosion resistance of a tin-coated steel base sheet which consists essentially of electrolytically coating a steel base sheet with a thin, first layer of tin to produce a tin-coated, steel base sheet, having a coating weight, per side, of tin within the range of about .04 to .08 pound per base box, fusing said first layer of tin, quenching the tin-coated, steel base sheet, applying to the tin-coated surfaces of the base sheet a chemical etchant characterized by its ability to rapidly react with iron and iron compounds and without deleteriously affecting said tin coated surfaces and selected from the group of acids consisting of hydrofluoric, fluophosphoric, sulfuric and hydrochloric, and coating the tin-coated steel base sheet with a second layer of tin.

4. A method for reducing the porosity of tin coating and increasing the corrosion resistance of a tin-coated steel base sheet which consists essentially of electrolytically coating a steel base sheet with a thin, first layer of tin to produce a tin-coated, steel base sheet, having a coating weight, per side, of tin within the range of about .04 to .68 pound per base box, fusing said first layer of tin, quenching the tin-coated steel base sheet applying to the tin coated surfaces of the base sheet an inorganic acid chemical etchant characterized by its ability to rapidly react with iron and iron compounds and without deleteriously affecting said in-coated surfaces and selected from the group of acids consisting of hydrofluoric, fluophosphoric, sulfuric and hydrochloric, and coating the tin-coated steel base sheet with a second layer of tin.

5. A method in accordance with claim 4 in which said etchant is hydrofluoric acid.

6. A method in accordance with claim 4 in which the tin-coated sheet coated with a second layer of tin is heated to an elevated temperature suflicient to fuse said tin.

7. A method for reducing the porosity of tin coating and increasing the corrosion resistance of a tin-coated steel base sheet strip which consists essentially of pickling said strip, cleaning said strip by immersion in a dilute aqueous solution of sulfuric acid followed by immersion in a dilute aqueous solution of sodium hydroxide, electrolytically coating said strip with a thin, first layer of tin to produce a tin-coated, steel base strip, rinsing said tin-coated strip, heating said tin-coated strip to an elevated temperature sufficient to fuse said first layer of tin, quenching the tin-coated steel base sheet strip, immersing said strip in a dilute aqueous solution of hydrofluoric acid without deleteriously affecting the tin-coated surfaces, rinsing said strip, electrolytically coating said tin coated strip with a second layer of tin to provide a double-coated, tin plate strip.

8. A method in accordance with claim 7 in which said thin first layer has a coating weight of less than about .1 pound of tin per base box.

9. A method in accordance with claim 7 in which said double-coated, tin plate strip is heated to a temeprature within the range of about 450 to 455 F. to fuse the tin coating.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Blum et al.: Principles of Electroplating and Electroforming, 2nd edition, 1930, pages 149-150. 

1. A METHOD FOR REDUCING THE POROSITY OF TIN COATING AND INCREASING THE CORROSION RESISTANCE OF A TIN-COATED STEEL BASE SHEET WHICH CONSISTS ESSENTIALLY OF COATING A STEEL BASE SHEET WITH A THIN, FIRST LAYER OF TIN TO PRODUCE A TIN-COATED, STEEL BASE SHEET, FUSING SAID FIRST LAYER OF TIN, QUENCHING THE TIN-COATED, STEEL BASE SHEET, APPLYING TO THE TIN-COATED SURFACES OF THE BASE SHEET A CHEMICAL ETCHANT CHARACTERIZED BY ITS ABLILITY TO RAPIDLY REACT WITH IRON AND IRON COMPOUNDS AND WITHOUT DELETERIOUSLY AFFECTING SAID TIN-COATED SURFACES AND SELECTED FROM THEORIC, GROUP OF ACIDS CONSISTING OF HYDROFLUORIC, FLUOPHOSPHORIC, BASE SHEET WITH A SECOND LAYER OF TIN. 